Just what constitutes a dewpond ?
Like so many areas of study there are probably as at least as many definitions
as there are people expressing an opinion - past and present.

This one would
cover most ponds that are generally accepted as Dew Ponds:Royal
Commission on Historic Monuments Dewpond:- A shallow pond, often
artificial, fed by the condensation of water from the air, occurring on high land
which has no other adequate water supply.

Some thoughts from a selection of
dictionaries:-Longman - a shallow pond, natural or artificial, fed by condensation, not
by a spring.Chambers - a hollow supplied with water by mist
Also: mist pond, sheep pond.